The present invention relates to a servo-assisted steering arrangement for an element pivotable about a steering shaft, comprising, in a hydraulic circuit, a first hydraulic pump driven by a manual drive means, at least two double-acting hydraulic piston cylinder devices connected in tie hydraulic circuit, each having a cylinder chamber on either side of the respective pistons, said piston cylinder devices being connected between the pivotable element and another element and a second hydraulic pump coupled into the hydraulic circuit, said second hydraulic pump being driven by a drive motor, the hydraulic circuit being divided into two, at least essentially mutually separated first and second partial circuits with a first hydraulic pump in the first partial circuit and a second hydraulic pump in the second partial circuit.
GB 2 159 482, for example, discloses a servo-assisted steering arrangement of the above type for pivoting an outboard motor about a vertical steering axis. The steering arrangement comprises a pair of piston cylinder devices, which are mounted between the ends of a steering arm joined to the engine propeller rig and mounting brackets on the boat hull. The piston rod of one of the piston cylinder devices is mechanically joined to a valve slide in a control valve, the valve housing of which is joined to a steering arm connected to the propeller rig. The control valve thus forms a mechanical connection between the piston rod of the first piston cylinder device and the propeller rig. The cylinder chambers on either side of the associated piston communicate with a steering wheel pump, which, when the steering wheel is turned, pumps hydraulic fluid to one or the other cylinder chamber, depending on the rotational direction of the steering wheel. The circuit, including the steering wheel pump and the first piston cylinder device, forms a low pressure circuit which is separate from a high-pressure circuit in which the control valve is coupled between a motor-driven hydraulic pump and the second piston cylinder device.
When the steering wheel is turned, oil is pumped into one cylinder chamber in the associated piston cylinder device, and at the same time the opposite cylinder chamber is drained. Initially, this leads to a displacement of the valve slide from its closed neutral position to one of its open lateral positions, in which a communication is established between the motor-driven pump and the opposite cylinder chamber in the second piston cylinder device, which leads to displacement of the piston in the opposite direction, which in turn results in the valve housing being displaced in the same direction as the valve slide during the steering movement. As long as the steering wheel is turned, the slide and the housing move together with the slide in the open position. When the turning of the steering wheel and the slide movement stops, the valve closes after a short displacement of the valve housing relative to the slide. The system described thus has a mechanical feedback, which requires that the control valve be movable together with the piston rod of the associated piston cylinder device. One disadvantage of a system with this function, i.e. initial turning of the steering wheel only takes up play and does not result in any steering forces, is that the driver will experience a marked looseness in the system when turning the steering wheel. Another disadvantage is that the total length of the piston cylinder device and the control valve makes it necessary that the distance between the mountings in the boat hull and in the propeller rig be relatively large- which makes it impossible to use the service device described together with certain marine drive units, e.g. an outboard drive unit of the type shown in SE 501 147 (U.S. Pat. No. 5,562,508).
The purpose of the present invention is to achieve a servo-assisted steering device of the type described by way of introduction, which is experienced as being virtually without play and which permits placement of the control valve independently of the placement of the piston cylinder device.
This is achieved to the invention by virtue of the fact that the partial circuits are so connected to the control valve means and to their individual pair of cylinder chambers that the flow in the first partial circuit is directed to one cylinder chamber of the associated pair of cylinder chambers via the control valve means, which at a predetermined pressure open the communication between the second hydraulic pump and one of the cylinder chambers of the associated pair of cylinder chambers, so that the pistons in the respective piston-cylinder devices are displaced in a direction dependent on the flow direction in the first partial circuit.
By controlling the flow from the steering wheel pump parallel to a steering cylinder and to the control valve, a maneuver pressure is achieved immediately in the cylinder striving to pivot the pivotable elements coupled to the cylinders. No play arises on the order of magnitude which is unavoidable in the described known system, in which the hydraulic cylinder coupled to the steering wheel pump only steers a slave cylinder. The servo-assisted steering arrangement according to the invention therefore provides the driver with a better steering feeling and control over the boat. Since the feedback between the steering cylinders, the control valve and the steering wheel pump is completely hydraulic, the control valve can be mounted wherever desired in the boat, which means that it will not, as with the control valve in the described known servo arrangement encroach on the space available for the steering cylinders.
One particular example of a control valve suitable for the steering arrangement according to the invention comprises a valve housing, a valve slide which is displaceable in said housing and which is spring-biased towards a neutral position, in which it breaks the communication between the inlet to and the outlet from the valve housing and a control piston which is joined to the valve slide and is displaceable in a cylinder in the valve housing, said cylinder having, on opposite sides of the control piston, cylinder chambers, each having an inlet and an outlet so that a pressure in either cylinder chamber strives to displace the control piston and thus the valve slide to one of two open positions.
A control valve of this type, which, in a control circuit for a steering device according to the invention, permits manual emergency steering in the event of pressure failure on the high pressure side, is characterized in that the valve slide in one of said two positions establishes communication between a valve housing inlet intended to be connected to the high pressure side in a hydraulic circuit, and one of two connections intended to be connected to individual pressure medium-actuated devices, at the same time as communication is established between the second connection and an outlet from the valve housing intended to be connected to the low pressure side of the hydraulic circuit, non-return valve means arranged in the valve housing permitting, if there is pressure failure on the high pressure side, flow from the low pressure side to the high pressure side when the pressure on the low pressure side exceeds the pressure on the high pressure side.